In recent years, GaN-based compound semiconductor materials have been attracting attention as semiconductor materials for use in short-wavelength light-emitting devices. The GaN-based compound semiconductors are formed on a substrate of a sapphire single crystal, various other oxides or Group III-V compounds by the MetalOrganic Chemical Vapor Deposition (MOCVD) method, Molecular Beam Epitaxy (MBE) method or the like.
The sapphire single crystal substrate, despite more than 10% difference in lattice constant from GaN, is finding popular acceptance because it is enabled to form thereon a good nitride semiconductor owing to the formation of a buffer layer of AlN or AlGaN. When a sapphire single crystal substrate is used, an n-type semiconductor layer, a light-emitting layer and a p-type semiconductor layer are stacked thereon sequentially in the order mentioned. Since the sapphire single crystal substrate is an insulator, the component structure thereof generally allows the existence of a positive electrode formed on the p-type semiconductor layer and a negative electrode formed on the n-type semiconductor layer. It comes in the two kinds, namely the face-up system that extracts light from the p-type semiconductor side by using a transparent electrode of Indium Tin Oxide (ITO), for example, as a positive electrode and the flip-chip system that extracts light from the sapphire substrate side by using a high-reflectance film of Ag, for example, as a positive electrode.
Though the sapphire single crystal substrate is generally finding a wide acceptance as described above, it entails several problems because it is an insulator.
The first problem is that, since the light-emitting layer is removed as by etching till the n-type semiconductor layer is exposed for the purpose of forming a negative electrode on the n-type semiconductor layer, the area of the light-emitting layer is decreased by the portion allocated for the formation of the negative electrode and the output is proportionately degraded.
The second problems is that, since both the positive electrode and the negative electrode exist on the same surface side of the substrate, the flow of electric current takes place inevitably in the horizontal direction and the portion of high current density is consequently formed locally, with the result that the device will generate heat.
The third problem is that, since the sapphire single crystal substrate has a low degree of thermal conductivity, the generated heat is not diffused and the temperature of the device is inevitably increased.
For the purpose of solving the problems enumerated above, the method for fabricating a nitride semiconductor light-emitting device by bonding an electroconductive substrate to a stacked body resulting from sequentially stacking an n-type semiconductor layer, a light-emitting layer and a p-type semiconductor layer in the order mentioned on a sapphire single crystal substrate, then removing the sapphire single crystal substrate and having a positive electrode and a negative electrode disposed on the upper and lower sides has been disclosed (refer to Japanese Patent No. 3511970).
The bonding of the electroconductive substrate entails various problems, such as the fluctuation of the thermal expansion coefficient of the substrate by the strength of bonding and the temperature of bonding and the increase of the resistance in the interface of bonding.
For the sake of heightening the strength of bonding, the method for bonding identical semiconductor devices after making the directions of crystal axes thereof coincide has been disclosed (refer to JP-A HEI 6-296040).
When this method is applied to a GaN-based semiconductor light-emitting device, it is necessary that a monocrystalline or polycrystalline substrate having all crystal faces gather in a uniaxial direction possessing electroconductivity be used as the substrate intended for bonding. When a silicon substrate is used, however, while it is advantageous to use a (111) face for the sake of heightening the bonding property of the silicon substrate with a GaN-based semiconductor device having an orientation of (00•1), it is actually difficult to heighten the bonding property because the length α/√−2 of one side corresponding to the Si (111) face is 3.84 Å, a magnitude deviating by 22% from the lattice constant a of GaN which is 3.16 Å.
The GaN-based semiconductor light-emitting device is ordinarily formed on a sapphire single crystal substrate by using the MOCVD method. When it is bonded at a high temperature (in the neighborhood of 300° C.) with a eutectic alloy, such as AuSn, a large difference in thermal expansion coefficient between the sapphire single crystal substrate and the bonding substrate (electroconductive substrate) results in generating thermal stress and preventing the bonding from successfully proceeding. As regards this problem, the idea of choosing Cu—W, for example, which is substantially equal in thermal expansion coefficient to the sapphire single crystal substrate for use in the bonding substrate has been disclosed (refer to JP-A 2004-266240).
This method, however, entails such problems as limiting the kind of substrate to be used for the bonding substrate and betraying vulnerability to temperature owing to the use of a eutectic metal for the bonding.
For the purpose of avoiding the difference in thermal expansion coefficient between the two substrates, the method of implementing the bonding in the neighborhood of normal room temperature has been disclosed (refer to JP-A 2004-337927).
Since this method cleans and activates the bonding surfaces by irradiating them with an inert gas ion beam, for example, in vacuum in the neighborhood of normal room temperature, it is effective in coping with the problem of the thermal expansion coefficient of the substrate and the problem of the increase of resistance of the interface of bonding. In the case of the GaN-based semiconductor light-emitting device, however, this method used alone cannot obtain sufficient bonding strength.
This invention has been achieved in view of the state of affairs mentioned above and is aimed, in producing a GaN-based semiconductor light-emitting device by bonding an electroconductive substrate, such as a silicon substrate, to a stacked body resulting from stacking GaN-based semiconductors on a substrate and then removing the substrate on the stacked body side, at providing a GaN-based semiconductor light-emitting device capable of heightening bonding strength and amply lowering resistance component on the bonding interface and a method for the fabrication thereof.